1. Field of the Invention
This invention relates to a confocal microscope and, more particularly, to a confocal microscope with a motorized scanning table for moving a sample perpendicularly to the optical axis of the microscope.
2. Discussion of Prior Art
A confocal microscope with a motorized scanning table to move a sample perpendicularly to the optical axis of the microscope is known from U.S. Pat. No. 5,239,178. Furthermore, the microscope has a light source array in a plane conjugate to the focal plane of an objective, and a detector array with numerous light-sensitive elements, also in a plane conjugate to the focal plane of the microscope objective. The movement of the specimen perpendicularly to the optical axis of the microscope takes place primarily in the microscopic region in order to increase the resolution, otherwise defined by the raster spacing of the light source array, perpendicular to the optical axis.
With this confocal microscope, sensing large object fields that are substantially greater than the visual field imaged by the objective is only possible to a limited extent. A series of individual images of the object must be recorded. Between each individual image, the object must be displaced over a path length corresponding to the image field diameter.
A Nomarski microscope (not confocal) is designed for taking and storing corresponding series of images, and is described, for example, in European Patent EP 0 444 450-A1. Since this Nomarski microscope is not confocal, it has only a small resolution in the direction of the optical axis. Furthermore, this microscope is much too slow when the image data in a large number of image fields must be sensed. The sensing of large object fields in the shortest possible time, with high resolution, is imperative in inspection equipment used in production processes, for example, in the semiconductor industry or in LCD production.
A microscope used for wafer inspection, also not confocal, is described in U.S. Pat. No. 5,264,912. In it, filtering takes place in the Fourier plane of the objective. The transmission characteristic of the spatial filter in the Fourier plane corresponds to the inverse diffraction figure of the integrated circuit (IC) that is being produced. Consequently, the filter transmits light only when the diffraction image of the momentarily imaged IC deviates from the diffraction image of the reference IC, and it can be concluded that the structure of the observed IC deviates from the reference structure. In this microscope, a CCD array or, alternatively, a high speed multiple output time delay integration (TDI) sensor is provided as the light detector. However, the reason for using a TDI sensor is not stated. Furthermore, because of the non-confocal arrangement, this microscope also has only a small resolution in the direction of the optical axis.
Furthermore, U.S. Pat. No. 5,365,084 includes an arrangement for inspecting a running length of fabric during its manufacture, in which a TDI sensor is used, synchronized with the motion of the length of fabric. However, such a video inspection device cannot be considered for inspecting semiconductors in a production process, because of its low resolution both in the direction of the optical axis and perpendicular to the optical axis.
The object of the present invention is to provide an arrangement that can be used for the optical inspection of semiconductors in the production process. With this arrangement, a further object is to achieve a sufficient resolution both in the direction of, and also perpendicular to, the optical axis. At the same time, an object is to sense large image fields in the shortest possible time. These objects are achieved by a confocal microscope including:
A motorized scanning table for moving an object at right angles to the optical axis of the microscope;
A diaphragm array in a plane that is conjugate to the focal plane of the microscope objective;
A sensor array following the diaphragm array in an observation direction with a plurality of photosensitive elements, charge storage elements associated with the photosensitive elements, and a device for displacing charges stored in the charge storage elements from one storage element to another storage element; and
A synchronizing unit for effecting displacement of the charges corresponding to motion of an image point of an object point in a plane of the sensor array.
The arrangement according to the invention is a confocal microscope with a motorized scanning table to move the specimen perpendicularly of the optical axis of the microscope. It has a diaphragm array with numerous light transmitting regions, so-called pinholes, in a plane that is conjugate to the focal plane of the microscope objective. The diaphragm array is followed by a sensor array that has numerous photosensitive elements. Each photosensitive element is associated with a charge storage element. Furthermore, the sensor array has a device for displacing the charges stored in the charge storage elements from one storage element to another storage element, as in the case in the so-called TDI sensors. Furthermore, a synchronizing unit is provided that effects displacing charges corresponding to the movement of the image point of a specimen point in the plane of the sensor array.
In the confocal microscopic arrangement, high resolution both in the direction of the optical axis and perpendicular to the optical axis, which is usual for confocal microscopes, is attained. The resolution that can be attained by using a strong magnifying objective, for example, one having a magnification of 20-120 times, is sufficient for semiconductor inspection. By using a diaphragm array, and the numerous parallel confocal beam paths associated with the diaphragm array, a number of object positions is sensed that correspond to the number of pinholes in the diaphragm array. By synchronizing the displacement of the charges in the sensor array corresponding to the motion of the image point of an object point, the measurement takes place while the sample is in motion. Preferably, the motion of the sample takes place along linear paths that extend over the complete length of the sample in the direction of motion. For sensing large, two-dimensional surfaces, corresponding linear paths can be combined in a meander form. Short acceleration or deceleration segments, during which no signal recording takes place, occur respectively at the start and at the end of each linear path. Between these acceleration and deceleration segments, the motion of the sample is uniform, so that the movement of charge between the storage elements of the sensor array and the motion of the image point on the sensor array are mutually synchronized.
In order to produce the parallel confocal beam paths, a light source array that has numerous mutually spaced-apart light sources is arranged in a plane conjugate to the focal plane of the objective. The positions of the individual light sources are conjugate to the positions of the transparent regions of the diaphragm array. Corresponding light source arrays can be formed in different ways. The simplest variant results when the diaphragm array is arranged in a common portion of the illumination and observation beam paths, and the diaphragm array is illuminated from the back. However, this simple arrangement has a disadvantage, in that a substantial portion of the illuminating light is reflected at the back side of the diaphragm array and thus produces a strong signal background on the sensor array. Such a strong signal background can be prevented by providing two separate diaphragm arrays, one in the illuminating beam path and the other in the observation beam path or measuring beam path. The diaphragm array in the illumination beam path is then again illuminated from the back. For a more effective use of light, the diaphragm array in the illumination beam path can be preceded by a lens array, as described in U.S. Pat. No. 5,239,178. In an alternative to using diaphragm arrays illuminated at the back, the light source array can also be formed by light-conducting fibers with their end surfaces arranged in an array. Likewise, as an alternative to a lens array, a correspondingly constructed diffractive element may be used.
As the sample is scanned, the diaphragm array, the light source array, and the sensor array are at rest. All three components are mutually stationary.
Preferably, the sensor array is a two-dimensional array of photosensitive elements and charge storage elements associated with the photosensitive elements, with numerous columns arranged parallel to each other. The direction of the columns is then defined by the direction in which the charges are displaced between the charge storage elements. On the one hand, the light source array and diaphragm array, and on the other hand, the sensor array, are arranged relative to each other so that at least one transparent region of the diaphragm array is imaged on each of the mutually parallel columns of the sensor array.
TDI sensors may be used as the corresponding sensor array. To the extent that such TDI sensors have light-insensitive regions between the photosensitive surfaces, these can be arranged, and the imaging between the diaphragm array and the sensor can be chosen so that the transparent regions of the diaphragm array are exclusively imaged on the photosensitive regions.
The transparent regions of the diaphragm array are formed, corresponding to the direction of motion of the scanning table and to the imaging ratio between the object plane and the diaphragm array, so that the paths of the images of all the transparent regions, closely fill, preferably without a gap, a portion of the focal plane, while maintaining the confocal filtering. With linear, one-dimensional scanning of the object, the image data for a strip whose width corresponds to the width of the image section sensed perpendicularly to the direction of motion is sensed completely confocally, without micro-movements perpendicular to the direction of motion required. For this purpose, the transparent regions of the diaphragm array may be arranged in the form of a two-dimensional rhombic grid. The midpoint of each transparent region then corresponds to the position of the theoretical grid point. However, it is particularly advantageous to arrange the transparent regions of the diaphragm array in the form of a rectangular grid, the grid axes of which are rotated relative to the linear direction of motion. Such a rectangular geometry confers advantages when the light source array is formed in the form of a fiber illumination, a lens array, or as a diffractive element producing a corresponding illumination.
Preferably, a particularly advantageous sensor array has several mutually independent two-dimensional partial sensor arrays that are arranged one behind the other in the column or stage direction, and that are respectively offset, perpendicularly to the column direction or stage direction, by a distance xcex94=d/n from each other, where d is the spacing of the individual sensors perpendicularly to the column direction and n is the number of two-dimensional partial arrays. Such an offset arrangement of several two-dimensional sensor arrays has an image field that is larger by the number of two-dimensional arrays in anamorphotic imaging of the diaphragm array on the sensor array, in contrast to an arrangement of a single sensor array with the same number of photosensitive elements, so that a correspondingly large signal/noise ratio results.